In recent, hot briquette iron (may be referred to as “HBI” hereinafter) has attracted attention as a raw material to be charged in a blast furnace which can cope with problems of both the recent tendency to higher tapping ratio operations and reduction of CO2 emission (refer to, for example, Non-patent Document 1).
However, conventional HBI is produced by hot forming of so-called gas-based reduced iron (reduced iron may be abbreviated to “DRI” hereinafter) which is produced by reducing fired pellets with high iron grade, which is used as a raw material, with reducing gas produced by reforming natural gas in a countercurrent heating-type reducing furnace such as a shaft furnace or the like. Therefore, conventional gas-based HBI is used as a raw material alternative to scraps in electric furnaces, but has a problem in practical use because of its high cost as a raw material for blast furnaces.
On the other hand, there has recently been developed a technique for producing so-called coal-based DRI by reducing a low-grade iron raw material with agglomerates incorporated with a carbonaceous material, which contain inexpensive coal as a reductant, in a high-temperature atmosphere of a radiation heating-type reducing furnace such as a rotary hearth furnace or the like, and practical application of the technique has been advanced (refer to, for example, Patent Documents 1 and 2).
However, the coal-based DRI is produced using a carbonaceous material incorporated as a reductant and thus has high porosity and a high content of residual carbon as compared with gas-based DRI. Therefore, the coal-based DRI has lower strength. Therefore, under the present conditions, in order to provide coal-based DRI with strength enough to resist charging in a blast furnace, the amount of the carbonaceous material incorporated is decreased to extremely decrease the residual C content in DRI, and strength is secured even by the sacrifice of metallization (refer to FIG. 3 of Non-patent Document 2). In addition, like the conventional gas-based DRI, the coal-based DRI is easily re-oxidized, and thus the coal-based DRI is unsuitable for long-term storage and long-distance transport.
Therefore, it is thought that like the conventional gas-based DRI, coal-based DRI is briquetted (i.e., to produce HBI) for the purpose of imparting higher strength and reoxidation resistance (weather resistance).
However, the briquetting has a problem in temperature control. Reduced iron discharged from a reducing furnace is at a high temperature, for example, about 750° C. to 900° C. in a current gas-based DRI production method using a countercurrent heating reducing furnace and about 1000° C. to 1100° C. in a coal-based DRI production method using a radiation heating-type reducing furnace. When such high-temperature reduced iron discharged from a reducing furnace is supplied in a hot state to a briquetting machine without substantially being cooled like in the present gas-based DRI production method, there occur various problems, for example, that the temperature of the reduced iron exceeds the limit of heat resistance of a briquetting roll and that the reduced iron is fixed in a pocket of the briquetting roll and is not easily separated.
A conceivable method for solving the problems include cooling, to some extent, high-temperature reduced iron discharged from a reducing furnace and then hot-forming the iron. However, when the reduced iron is excessively cooled, the reduced iron is hardened to worsen formability, thereby causing problems, such as the need to increase forming pressure, the occurrence of cracks in produced HBI, and the like.
Further, Patent Documents 3 to 5 disclose cooling methods using a rotary kiln, but any one of the methods aims at cooling high-temperature reduced pellets to finally room temperature, and the documents do not disclose means for solving the problems.
Non-Patent Document 1: Y Ujisawa, et al. Iron & Steel, vol. 92 (2006), No. 10, p. 591-600
Non-Patent Document 2: Takeshi Sugiyama et al. “Dust Treatment by FASTMET (R) Process”, Resource Material (Shigen Sozai) 2001 (Sapporo), Sep. 24-26, 2001, 2001 Autumn Joint Meeting of Resource Materials-Related Society (Shigen Sozai Kankeigaku Kyokai)
Patent Document 1: Japanese Unexamined Patent Application Publication No 11-279611
Patent Document 2: Japanese Unexamined Patent Application Publication No 2001-181721
Patent Document 3: Japanese Examined Patent Application Publication No 7-42523
Patent Document 4: Japanese Unexamined Patent Application Publication No 2002-38211
Patent Document 5: Japanese Unexamined Patent Application Publication No 2001-255068